Cordless telephones which are capable of operating on more than one channel are well known. These products range in capability from a relatively simple two-channel crystal controlled design to the more sophisticated frequency synthesized techniques, which permit either manual channel changing or full scanning operation. In spite of its adding a significant cost burden to the product, multiple channel operation is the only effective method for dealing with channel interference that may be caused by other users of cordless telephones in the local vicinity or other electro magnetic sources of interference. This is particularly significant in highly urbanized areas where many cordless telephones and other electronic devices are used.
The functioning of cordless telephones in the United States is constrained by having to operate in one of ten channels within a relatively narrow frequency band width set by the FCC. In most urban environments, this may result in interference on one or more channels. Existing techniques for dealing with interference range from manually changing the channel of operation to using automatic frequency selection, depending on the individual model.
Manual channel selection begins with the recognition by the user that the symptoms being experienced are in fact interference related. In many instances, this can be far from obvious. Symptoms such as misdialing can be caused by interference, even in those models that have digital security codes. A trial-and-error process is typically used to find a new channel that is not subject to interference. For a two-channel product this can be relatively brief but has a corresponding low probability of success, particularly in urban environments where there is a very good chance that any two of the ten available channels will be used simultaneously by other cordless units in the neighborhood. For a ten-channel product, the chances of eventually finding a clear channel are good, however, the process of finding a clear channel can be tedious.
An improvement is realized in products which automatically scan all ten channels every time the cordless telephone is used. This makes the entire spectrum available to the user and ensures that all communication will at least start on a clear channel. Significant problems still remain, however. Users of this type of system will immediately notice annoying delays in common operation. Lapses of several seconds may occur between placing the unit in the "Talk" mode and obtaining a dial tone or being able to begin a conversation. The principal reason for this is simply that scanning ten channels takes time. To overcome this serious limitation, a more selective channel scanning method has been proposed in the prior art.
In the patent to Yamagato et al., U.S. Pat. No. 4,768,219, the cordless telephone units, that is the handset and the base, independently scan all the channels during the standby mode and store in their respective memories three of those channels that have interference. When an outgoing call is thereafter to be made, or an incoming call is received, the unit skips those channels that are stored in either the memories of the base or handset and searches for a vacant channel, whereupon communication is established between the handset and the base.
Though the method disclosed in Yamagato is a definite improvement over scanning all ten channels, it too has serious draw backs. Even though the handset scans the frequency spectrum almost continuously, it begins its actual search for a vacant channel only when an incoming call is received or when an outgoing call is about to be made. Depending on the distribution of the vacant channels, this could cause considerable delay. Also, it does not discriminate between a channel that experiences interference less frequently than another channel. In most urban environments, it is not unrealistic to expect some channels to be used very frequently, some to be used sporadically and some very rarely. The device of Yamagato is therefore perfectly capable of selecting the same channel that a neighbor's cordless phone uses, as long as it is not in use at the exact moment of scanning, even if the other six channels are completely unutilized.
Another serious disadvantage of the Yamagato device is that the handset and the base may each attempt to communicate on channels that have been marked by the other as being noisy. This could happen because the handset and the base keep different channel storage areas (A1, A2, A3 in the handset and A1, A2, and A3 in the base). Also, the system's continuous scanning results in the reduction of handset battery life. This is due to the fact that the handset receiver must be powered up 100% of the time. It does not learn or adapt to the channel usage patterns of its environment.